Have you played Doom 3? If so, you should be able to run things at about the same level, as Q4 is based off the same engine. It's got a few extras, which some people are saying has it lagging a little in some area's, but pretty much, it's the same thing performance-wise.
I'm curious why you have a 6600GT listed under my computer but you want to play it with a 9800Pro? Someone elses computer? Either way I'd say high on 800x600 for starters and see how that goes, if it plays well step it up to 1024x768 but I wouldn't try putting on any AA or AF if I were you.